The present invention relates to an output impedance varying circuit for adjusting the output impedance of a photo detector IC in an apparatus for optically recording and playing back information.
In an apparatus for optically recording and playing back information, such as an optical disk drive or a magneto-optical disk drive, an optical pickup for converting light reflected from a disc into an electrical signal is composed of a lens for collecting mainly light, a photo detector IC (PDIC) which is a semiconductor device for converting an optical signal into a voltage, and a flexible printed circuit board (FPC) for establishing a connection from the PDIC to a signal processing board of the main body.
The output of the PDIC is input into the signal processing board of the main body via the FPC. The FPC can be expressed as an equivalent circuit composed of a coil and a capacitor. Therefore, in the input frequency characteristics of the signal processing board, peaking caused by the transfer characteristics of the circuit formed of the coil and the capacitor of the FPC appears. This peaking goes into an internal circuit of the PDIC through a parasitic element, a common impedance, or the like in the PDIC to change the frequency characteristics of the PDIC or to cause an operational amplifier in the PDIC to oscillate.
In a known transistor switching circuit, in order to adjust the frequency characteristics of an amplifier, a peaking circuit is electrically connected or disconnected with the amplifier according to a control signal (see Japanese Laid-Open Publication No. 62-264721).
To prevent peaking, an output resistor may be additionally provided in the PDIC. Specifically, in order to prevent peaking, the resistance value of the output resistor is optimized in accordance with the values of the coil and capacitor of the FPC connected with the PDIC.
Nevertheless, if the length of the FPC is changed due to new developments of optical pickups or the like, the inductance value and capacitance value of the FPC are changed. And in the case of a conventional resistance value, peaking occurs again or the input frequency characteristics of the signal processing board decrease to lower the level in the signal bandwidth.
If an output resistor is provided not in the PDIC but in the FPC, the cost of the optical pickup itself increases, while the increased number of components in the FPC results in increase in the size of the optical pickup itself.